Transient Electroluminescence Research Articles

Stable pure-green organic light-emitting diodes toward Rec.2020 standard

Manipulating dynamic behaviours of charge carriers and excitons in organic light-emitting diodes (OLEDs) is essential to simultaneously achieve high colour purity and superior operational lifetime. In this work, a comprehensive transient electroluminescence investigation reveals that incorporating a thermally activated delayed fluorescence assistant molecule with a deep lowest unoccupied molecular orbital into a bipolar host matrix effectively traps the injected electrons. Meanwhile, the behaviours of hole injection and transport are still dominantly governed by host molecules. Thus, the recombination zone notably shifts toward the interface between the emissive layer (EML) and the electron-transporting layer (ETL). To mitigate the interfacial carrier accumulation and exciton quenching, this bipolar host matrix could serve as a non-barrier functional spacer between EML/ETL, enabling the distribution of recombination zone away from this interface. Consequently, the optimized OLED exhibits a low driving voltage, promising device stability (95% of the initial luminance of 1000 cd m−2, LT95 > 430 h), and a high Commission Internationale de L’Éclairage y coordinate of 0.69. This indicates that managing the excitons through rational energy level alignment holds the potential for simultaneously satisfying Rec.2020 standard and achieving commercial-level stability.

Unraveling the Degradation Pathways in Deep Blue Phosphorescent OLEDs Depending on Charge Dynamics: Insights from Numerical Analysis and Magneto-Electroluminescence Characterization.

To analyze the lifetime difference based on the charge dynamics in the emitting layer (EML), we applied two electron transport layers (ETLs) with significantly different electron transporting characteristics to the same EML. Even with the same EML configuration, the device lifetime increased by approximately 4-fold, from 291 h to over 1000 h of LT50 (the time taken for the luminance to decrease to 50% of its initial value of 1000 cd/m2). Although trap/detrap of holes in the dopant molecules was observed through impedance spectroscopy, we found that the most significant difference in lifetime was caused by the quantity of electron current. Surprisingly, depending on the electron transporting layer, the primary bimolecular interaction in the EML (i.e., exciton-exciton, exciton-polaron interaction) dramatically changes even in the same EML configuration, which is theoretically analyzed by the numerical fitting of transient electroluminescence data and experimentally confirmed by magneto-electroluminescence (MEL) measurements. To the best of our knowledge, for the first time, the MEL measurements are demonstrated as a tool that can be utilized to intuitively discern the dominance of bimolecular interaction with respect to the operational stability of phosphorescent organic light-emitting diodes (PhOLEDs).

Deep Learning Prediction of Triplet-Triplet Annihilation Parameters in Blue Fluorescent Organic Light-Emitting Diodes.

The triplet-triplet annihilation (TTA) ratio and the rate coefficient (kTT) of TTA are key factors in estimating the contribution of triplet excitons to radiative singlet excitons in fluorescent TTA organic light-emitting diodes. In this study, deep learning models are implemented to predict key factors from transient electroluminescence (trEL) data using new numerical equations. A new TTA model is developed that considers both polaron and exciton dynamics, enabling the distinction between prompt and delayed singlet decays with a fundamental understanding of the mechanism. In addition, deep learning models for predicting the kinetic coefficients and TTA ratio are established. After comprehensive optimization inspired by photophysics, determination coefficient values of 0.992 and 0.999 are achieved in the prediction of kTT and TTA ratio, respectively, indicating a nearly perfect prediction. The contribution of each kinetic parameter of polaron and exciton dynamics to the trEL curve is discussed using various deep-learning models.

Purely Organic Room‐Temperature Phosphorescence Molecule for High‐Performance Non‐Doped Organic Light‐Emitting Diodes

Purely organic molecules with room‐temperature phosphorescence (RTP) are potential luminescent materials with high exciton utilization for organic light‐emitting diodes (OLEDs), but those exhibiting superb electroluminescence (EL) performances are rarely explored, mainly due to their long phosphorescence lifetimes. Herein, a robust purely organic RTP molecule, 3,6‐bis(5‐phenylindolo[3,2‐a]carbazol‐12(5H)‐yl)‐xanthen‐9‐one (3,2‐PIC‐XT), is developed. The neat film of 3,2‐PIC‐XT shows strong green RTP with a very short lifetime (2.9 μs) and a high photoluminescence quantum yield (72%), and behaviors balanced bipolar charge transport. The RTP nature of 3,2‐PIC‐XT is validated by steady‐state and transient absorption and emission spectroscopies, and the working mechanism is deciphered by theoretical simulation. Non‐doped multilayer OLEDs using thin neat films of 3,2‐PIC‐XT furnish an outstanding external quantum efficiency (EQE) of 24.91% with an extremely low roll‐off (1.6%) at 1000 cd m‒2. High‐performance non‐doped top‐emitting and tandem OLEDs are also achieved, providing remarkable EQEs of 24.53% and 42.50%, respectively. Delightfully, non‐doped simplified OLEDs employing thick neat films of 3,2‐PIC‐XT are also realized, furnishing an excellent EQE of 17.79% and greatly enhanced operational lifetime. The temperature‐dependent and transient EL spectroscopies demonstrate the electrophosphorescence attribute of 3,2‐PIC‐XT. These non‐doped OLEDs are the best devices based on purely organic RTP materials reported so far.

Purely Organic Room-Temperature Phosphorescence Molecule for High-Performance Non-Doped Organic Light-Emitting Diodes.

Purely organic molecules with room-temperature phosphorescence (RTP) are potential luminescent materials with high exciton utilization for organic light-emitting diodes (OLEDs), but those exhibiting superb electroluminescence (EL) performances are rarely explored, mainly due to their long phosphorescence lifetimes. Herein, a robust purely organic RTP molecule, 3,6-bis(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)-xanthen-9-one (3,2-PIC-XT), is developed. The neat film of 3,2-PIC-XT shows strong green RTP with a very short lifetime (2.9 μs) and a high photoluminescence quantum yield (72%), and behaviors balanced bipolar charge transport. The RTP nature of 3,2-PIC-XT is validated by steady-state and transient absorption and emission spectroscopies, and the working mechanism is deciphered by theoretical simulation. Non-doped multilayer OLEDs using thin neat films of 3,2-PIC-XT furnish an outstanding external quantum efficiency (EQE) of 24.91% with an extremely low roll-off (1.6%) at 1000 cd m‒2. High-performance non-doped top-emitting and tandem OLEDs are also achieved, providing remarkable EQEs of 24.53% and 42.50%, respectively. Delightfully, non-doped simplified OLEDs employing thick neat films of 3,2-PIC-XT are also realized, furnishing an excellent EQE of 17.79% and greatly enhanced operational lifetime. The temperature-dependent and transient EL spectroscopies demonstrate the electrophosphorescence attribute of 3,2-PIC-XT. These non-doped OLEDs are the best devices based on purely organic RTP materials reported so far.

Abnormal temperature-dependent transient electroluminescence spikes induced by the leakage of hole carriers in organic light-emitting diodes

The amplitude of the emission spike at the transient electroluminescence (TEL) falling edge is an important benchmark for evaluating the quantities of trapped charges existed in organic light-emitting diodes and often shows a normal temperature dependence which increases with the decreasing temperature. Surprisingly, an unreported abnormal temperature-dependent TEL spike was observed in this work. A series of experimental results relevant to the electroluminescence spectrum and TEL measurements demonstrate that this abnormal temperature-dependent behavior is induced by the leakage of hole carriers from the emission layer (EML) to an electron transport layer (ETL). After the voltage pulse is turned off, these holes already leaked into the ETL drift back toward the EML, subsequently engaging in radiative recombination with trapped electrons on guest molecules to generate a spike at the TEL falling edge. However, the drift process is hindered by the reduced carrier mobility of the ETL material with the decrease in temperature. As a result, the spike intensity weakens as the temperature decreases, which contradicts the conclusions reported in previous literatures. Therefore, this study not only leads to the reconsideration for the judgment criteria of the number of trapped charges but also provides valuable insight into the TEL research field of organic optoelectronic devices.

High Efficiency Triplet‐Triplet Fusion Blue Fluorescence OLEDs by introducing a “Hot Exciton” Efficiency Enhancement Layer

AbstractTriplet‐triplet fusion (TTF) can effectively transfer triplet excitons to singlet energy level and exhibit good stability, making it widely used in blue fluorescence organic light emitting diodes (OLEDs) for practical application. However, the efficiency of the resulting blue fluorescence OLEDs is still fundamentally limited. Herein, this work introduces a hot exciton efficiency enhancement layer (EEL) between TTF emissive layer and electron‐transporting layer for the first time. It can be found that the triplet excitons are greatly utilized by the efficient hot exciton EEL. As a result, a high efficiency blue fluorescence OLED is successfully developed, exhibiting a maximum external quantum efficiency of 15.2% at the luminance of 4068.0 cd m−2, achieving a breakthrough in the efficiency of blue TTF‐OLEDs. Transient electroluminescence (TrEL) measurements and detailed theoretical calculations confirm the key role of the effective hot exciton process from the high‐lying triplet energy level (Tn, n ≥ 2) to S1 via a reverse intersystem crossing (hRISC). Furthermore, the device lifetime is also greatly improved owing to the fast and efficient hRISC in the introduced hot exciton EEL. This work has important guiding significance for constructing high efficiency blue fluorescence OLEDs by introducing hot exciton molecules.

Study of pristine and degraded blue quantum dot light-emitting diodes by transient electroluminescence measurements

Limited stability of blue quantum dot light-emitting diodes (QLEDs) under current stress impedes commercialization. Multi-layer structures of the state-of-the-art blue QLEDs pose significant difficulty in the fundamental understanding of degradation mechanisms. Here, by applying transient electroluminescence measurements, we disentangle charge transport in both pristine and degraded blue QLEDs. By varying thicknesses of the charge transport layers and the emissive layer, respectively, we show that the charge transport in pristine QLEDs is primarily dominated by holes. Furthermore, the degradation of QLEDs under electrical stress is governed by the decrease of hole transport in the emissive quantum dot layer due to the formation of hole traps.

Aryl‐Bridged Thienonaphthalimides: Synthesis, Characterization and Optoelectronic Properties

AbstractThree small molecules consisting of thieno‐fused naphthalimide units connected via aromatic linkers were synthesized to study the effect of fused thiophene rings on the properties of naphthalimide‐based molecules. A protocol for the synthesis of thienonaphthalimide‐based molecules without prior preparation and use of thienonaphthalimide precursors and organoelement compounds (organolithium, organotin, or organoboron compounds) was developed. The protocol is based on double thienannulation of bis‐alkyne substrates with sodium sulfide. The thin‐film morphology, photophysical and electrochemical properties of the obtained small molecules were studied. Single‐layer non‐doped organic light‐emitting diodes based on fluorene‐bridged thienonaphthalimide dimer showed yellow emission with luminance of 2200 cd/m2, and current efficiency of 0.35 cd/A. Combination of transient electroluminescence and photo‐CELIV methods allowed to estimate electron and hole mobility in the synthesized compounds; both are in order of 10−6 cm2V−1 s−1.

Differences in Electron and Hole Injection and Auger Recombination between Red, Green, and Blue CdSe-Based Quantum Dot Light Emitting Devices.

Despite the significant progress that has been made in recent years in improving the performance of quantum dot light-emitting devices (QLEDs), the effect of charge imbalance and excess carriers on excitons in red (R) vs green (G) vs blue (B) QLEDs has not been compared or systematically studied. In this work we study the effect of changing the electron (e)/hole (h) supply ratio in the QDs emissive layer (EML) in CdSe-based R-, G-, and B-QLEDs with inverted structure in order to identify the type of excess carriers and investigate their effect on the electroluminescence performance of QLEDs of each color. Results show that in R-QLEDs, the e/h ratio in the EML is >1, whereas in G- and B-QLEDs, the e/h ratio is <1 with charge balance conditions being significantly worse in the case of B-QLEDs. Transient photoluminescence (PL) and steady state PL measurements show that, compared to electrons, holes lead to a stronger Auger quenching effect. Transient electroluminescence (TrEL) results indicate that Auger quenching leads to a gradual decline in the EL performance of the QLEDs after a few microseconds, with a stronger effect observed for positive charging versus negative charging. The results provide insights into the differences in the efficiency behavior of R-, G-, and B-QLEDs and uncover the role of excess holes and poor charge balance in the lower efficiency and EL stability of B-QLEDs.

Anthracene derivatives with strong spin-orbit coupling and efficient high-lying reverse intersystem crossing beyond the El-Sayed rule.

The attention to materials with hot exciton channel and triplet-triplet fusion (TTF) mediated high-lying reverse intersystem crossing (hRISC) has been raised for their ability to convert non-emissive 'dark' triplets into radiative singlet excitons. This spin conversion process results in high exciton utilization efficiency (EUE) that exceeds the theoretical limits. Notably, it is known that such spin conversion processes from the high-lying excited triplet to the singlet state are facilitated by the orthogonal orbital transition effect governed by the El-Sayed's rule. In this study, an anthracene derivative with indenoquinoline substituent 7,7-dimethyl-9-(10-(4-(naphthalen-1-yl)phenyl)anthracen-9-yl)-7H-indeno[1,2-f]quinoline (2MIQ-NPA) was synthesized and analyzed to investigate whether the hRISC process occurs in these molecules, even when the El-Sayed's rule is not followed. The hRISC channels of the emitter were fully unraveled through DFT calculations and experiments, which were quantitatively subdivided using transient electroluminescence measurements. The results showed that 2MIQ-NPA, which does not follow the El-Sayed's rule and has a relatively strong spin-orbit coupling matrix element of 0.116 cm-1 between the high-lying triplet state of T4 and the lowest singlet state of S1, effectively converted triplet excitons into singlet excitons with an EUE of 64.3%, contributed by a direct hot exciton channel of 19.2% and a TTF-mediated hot exciton channel of 15.1%. Despite the low outcoupling efficiency, the non-doped device with 2MIQ-NPA achieved an excellent device performance with an external quantum efficiency of 7.0%.

Stabilized Huang-Rhys factor in non-doped exciplex-based Firpic phosphorescent organic light emitting diodes via quantum well-like multiple emissive layer structure and investigation of carrier behaviors by transient electroluminescence

Stabilized Huang-Rhys factor in non-doped exciplex-based Firpic phosphorescent organic light emitting diodes via quantum well-like multiple emissive layer structure and investigation of carrier behaviors by transient electroluminescence

Effects of host-molecule transport polarities on the transient electroluminescence decay and efficiency roll-off of doped organic light-emitting diodes

Effects of host-molecule transport polarities on the transient electroluminescence decay and efficiency roll-off of doped organic light-emitting diodes

Mitigating the Trade-Off between Non-Radiative Recombination and Charge Transport to Enable Efficient Ternary Organic Solar Cells.

Ternary organic solar cells (OSCs) have attracted intensive studies due to their promising potential for attaining high-performing photovoltaics, whereas there has been an opening challenge in minimizing the open circuit voltage (Voc) loss while retaining the optimal carrier extraction in the multiple mixture absorbers. Here, we systemically investigate a ternary absorber comprised of two acceptors and a donor, in which the resultant Voc and fill factor are varied and determined by the ratios of acceptor components as a result of the unbalance of non-radiative recombination rates and charge transport. The transient absorption spectroscopy and electroluminescence techniques verify two distinguishable charge-transfer (CT) states in the ternary absorber, and the mismatch of non-radiative recombination rates of those two CT states is demonstrated to be associated with the Voc deficit, whilst the high-emissive acceptor molecule delivers inferior electron mobility, resulting in poor charge transport and a subpar fill factor. These findings enable us to optimize the mixture configuration for attaining the maximal-performing devices. Our results not only provide insight into maximizing the photovoltage of organic solar cells but can also motivate researchers to further unravel the photophysical mechanisms underlying the intermolecular electronic states of organic semiconductors.

Study on the exciton dynamic processes of exciplex host–guest system by transient magnetic field effects

Utilizing exciplex as the host and fluorescence emitter with dopant materials has been proved successfully to fabricate highly-efficient organic light-emitting diodes. Exciton evolution and energy transfer in this exciplex host–guest system are complex. Gaining insight into the electroluminescence (EL) mechanisms in exciplex-based devices is key for further optimizing device configuration. Here, we have investigated exciton dynamics in devices with exciplex as host and 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) as red fluorescence emitter. Two exciplexes, 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPY) doped 2,4,6-tris[3-(triphenylphosphine)phenyl]-1,3,5-triazine (POT2T), and 4,4′,4″-tris[3-methylphenyl(phenyl)amino]-triphenylamine (m-MTDATA) doped tris(8-hydroxyquinoline)aluminum(III) (Alq3), with different band energy are utilized as host materials. Combining the measurements of transient EL, transient photoluminescence and magnetic field effect (MFE), it is concluded that Dexter energy transfer, together with Förster resonance energy transfer, are confirmed in the pure fluorescence doped system. Meanwhile, it is found that DCJTB works with the hot excitons mechanism but not a traditional red fluorescence emitter as recognized previously. This work presents that the transient MFE is powerful for detecting excitonic dynamic processes in excipelx based host–guest EL systems.

P‐132: Carriers Transport Dynamics in Quantum‐Dot Light‐Emitting Diode by Transient Electroluminescence Analysis

The outstanding performance of quantum dots light emitting diodes (QLED), like high color purity, high brightness, high efficiency, and low fabrication cost, has attracted significant attention from academic researchers and industry. A great improvement has been established in QLED performance, but a few works focus on investigating the operation dynamic mechanism that is critical for understanding the device deterioration and further performance improvement. Here, we utilized transient electroluminescence (TREL) to investigate the carrier's transport dynamics. We believe this method based on TREL will contribute to understanding the carriers' transport, especially for hole transport and fabricating high‐performance QLED devices. Based on the analysis results, we also demonstrated a passive matrix (PM) electroluminescence display panel with high brightness and vivid color.

65‐3: Kinetic Pathways of Excitons and Polarons Governing Device Degradation in Exciplex‐Forming Co‐Host‐Based Organic Light‐Emitting Diodes

The exciplex‐forming cohost with thermally activated delayed fluorescence characteristic has been used for achieving a long operational lifetime and high efficiency in phosphorescent organic light‐emitting diodes (PHOLEDs). In this work, the intrinsic polaron dynamics in the device are comprehensively investigated in terms of the polaron and exciton behaviors through the numerical simulation with transient electroluminescence (EL). Furthermore, the degradation mechanism generated by bimolecular interaction is investigated by considering the reduction of the luminance and voltage rising. As a result, the polaron, exciton, and recombination coefficient could be quantified via numerical simulation. Based on this, it is revealed that suppressing the exciton‐polaron interaction is crucial to improving the long operational lifetime in exciplex‐forming cohost based PHOLEDs.

Understanding the Electroluminescence Mechanism of CdSe/ZnS Quantum-Dot Light-Emitting Diodes With a Focus on Charge Carrier Behavior in Quantum-Dot Emissive Layers

We analyzed the electroluminescence (EL) mechanisms of quantum-dot light-emitting diodes (QLEDs) with a focus on charge carrier trapping/detrapping. Multilayer quantum-dot (QD) emissive layers (EMLs) exhibit low void density and many QD surface traps, which provide high efficiency in the low bias voltage region; the efficiency gradually decreases in the high bias voltage region. The roles played by traps and charge carrier trapping/detrapping during QLED EL were revealed by transient EL and impedance spectroscopy analyses. Charge carrier trapping/detrapping of QLEDs with a multilayer QD EML in the low bias voltage region promoted charge injection, rapid initiation of EL, and high current efficiency. In contrast to the conventional view, traps improved QLED performance.

P‐105: Understanding Recombination Dynamics from Electroluminescence of Organic Light‐Emitting Diodes Using Artificial Intelligence

Distinguishing exciton and polaron dynamics in the electroluminescence (EL) of organic light‐emitting diodes (OLEDs) is difficult since the delayed EL originated from a combination of the excitonic process and slow recombination process. In this work, the recombination dynamics was extracted from the transient EL only via prediction of the recombination coefficient using artificial intelligence (AI). AI model with a high prediction accuracy (R 2 ) of 0.947 successfully analyzed the polaron dynamics of exciplex‐forming co‐host‐based OLEDs.

Substrate Dependence of CdSe/ZnS Quantum-Dot Light-Emitting Diodes: A Comparative Study between Rigid Glass and Flexible Plastic Substrates.

The purpose of this study was to investigate the effect of substrate characteristics on the performance of quantum-dot light-emitting diodes (QLEDs) with the aim of developing high-performance flexible QLEDs. Specifically, we compared QLEDs made with a flexible polyethylene naphthalate (PEN) substrate to those made with a rigid glass substrate, using the same materials and structure except for the substrates. Our findings indicate that the PEN QLED had a 3.3 nm wider full width at half maximum and a 6 nm redshifted spectrum in comparison to the glass QLED. Additionally, the PEN QLED exhibited a 6% higher current efficiency, a flatter current efficiency curve, and a 2.25-V lower turn-on voltage, indicating superior overall characteristics. We attribute the spectral difference to the optical properties of the PEN substrate, such as light transmittance and refractive index. Our study also revealed that the electro-optical properties of the QLEDs were consistent with the electron-only device and transient electroluminescence results, which suggests that the improved charge injection properties of the PEN QLED were responsible. Overall, our study provides valuable insights into the relationship between substrate characteristics and QLED performance, which can be used to develop high-performance QLEDs.